The present invention relates to a nonvolatile semiconductor memory device, more specifically, a nonvolatile semiconductor memory device using a resistance memory element having a plurality of resistance states of different resistance values and a method of writing into the same.
Recently, as a new memory device, a nonvolatile semiconductor memory device called RRAM (Resistance Random Access Memory) is noted. The RRAM uses a resistance memory element which has a plurality of resistance states of different resistance values, which are changed by electric stimulations applied from the outside and whose high resistance state and low resistance state are corresponded to, e.g., information “0” and “1” to be used as a memory element. The RRAM highly potentially has high speed, large capacities, low electric power consumption, etc. and is considered prospective.
The resistance memory element has a resistance memory material whose resistance states are changed by the application of voltages sandwiched between a pair of electrodes. As the typical resistance memory material, oxide materials containing transition metals are known.
The nonvolatile semiconductor memory device using the resistance memory element is disclosed in, e.g., U.S. Pat. No. 6,473,332 (herein after called Reference 1), Japanese published unexamined patent application No. 2005-025914 (herein after called Reference 2), Japanese published unexamined patent application No. 2004-272975 (herein after called Reference 3), Japanese published unexamined patent application No. 2004-110867 (herein after called Reference 4), A. Beck et al., Appl. Phys. Lett., Vol. 77, p. 139 (2000) (herein after called Reference 5), W. W. Zhuang et al., Tech. Digest IEDM 2002, p. 193 (herein after called Reference 6), and I. G. Baek et al., Tech. Digest IEDM 2004, p. 587 (herein after called Reference 6).
However, in the resistance memory element using the resistance memory material described above, the impedance of the cell much varies between the resistance value in the high resistance state and the resistance value in the low resistance state, which makes the impedance matching with outside circuits difficult.
For example, TiOx, which is a typical transition metal oxide, has the resistance value at 0.5 V varied by about 3 places between the high resistance state and the low resistance state. Accordingly, when the impedance in, e.g., the high resistance state is matched with that of an outside circuit, the impedance matching with the outside circuit in the low resistance state is largely broken, and reversely, when the impedance is matched with the outside circuit in the low resistance state, the impedance matching with the outside circuit in the high resistance state is largely broken. Consequently, in high speed operations, voltage pulses are reflected at the connection with the outside circuit, which makes it impossible to apply effectively sufficient voltages to the resistance memory element both in the low resistance state and the high resistance state.
When effectively sufficient voltages cannot be applied to the resistance memory element due to the impedance mismatching, the resistance states cannot be switched, which makes the writing and erasing impossible, and errors take place. To prevent this, the pulse width is elongated to make the voltage application time longer, but this lowers the operation speed.